One of the important and microscopic characteristics of the condensation process is that water vapor will not condense into liquid water very easily unless it condenses onto a foreign object such as the tiny hairlike structures on grasses or dust and pollen particles on windshields. In fact, on particularly dewy mornings, if you wait for the dew to evaporate you may find yellow stains on your windshield that are left as the liquid water evaporates leaving the pollen particles on which it originally condensed.

The formation of raindrops requires a similar collection of foreign objects upon which water vapor can condense. Such objects are known as cloud condensation nuclei and many naturally occurring substances can serve this role, including dust particles, smoke particles, salt particles, pollen grains, particulate matter from smokestacks, and naturally occurring aerosol particles.

Without these cloud condensation nuclei, the formation of cloud liquid water droplets, and eventually precipitation-sized particles (which are 1 million times more voluminous), would be considerably more difficult in our atmosphere. In that case, rain and snow would be rare, and life on the planet would be put at risk.

If you were out on Tuesday night sometime between 8 and 9 p.m., you might have seen the sky glow green. It wasn’t a St. Patrick’s Day mirage; it was the northern lights, also called aurora borealis, which appear as a diffuse glow or as overlapping curtains of greenish-white and sometimes red light in the sky.

Auroras are triggered when the sun ejects a cloud of gas, called a coronal mass ejection. It takes about two or three days for the charged particles in this gas to reach Earth.

Earth’s magnetic field deflects these particles toward the North and South Poles. When these charged particles collide with a molecule or atom they can excite the molecule, increasing their energy state. When these molecules or atoms shift back down to their normal energy states they emit light.

Auroras form between 60 and 250 miles above the Earth’s surface when these charged solar particles collide with two abundant constituents of our atmosphere: nitrogen and oxygen. Nitrogen emits pinkish or magenta light, while oxygen emits greenish light.

Most of the collisions occur near the poles, so the northern lights are usually seen at the higher latitudes of Canada and Alaska.

The northern lights were particularly visible from southern Wisconsin. It is not very common for the northern lights to reach this far south.

Last week, Sen. James Inhofe of Oklahoma, the current chairman of the Senate Environment and Public Works Committee, brought a snowball fashioned from accumulated snow in Washington, D.C., onto the floor of the United States Senate.

Inhofe used the snowball as proxy “evidence” of, in his words, the “hysteria on global warming.”

It is undeniable that the second half of this winter has been unusually cold over a large portion of North America, with an especially brutal combination of snow and cold over the eastern seaboard.

However, it is also true that this past winter (December/January/February) registered the smallest average areal extent of lower tropospheric cold air since records began in 1948. In fact, the past two winters, cold though they have been over North America, have set that record in back-to-back years.

This fact makes for interesting scientific work ahead. On the one hand, we hope to find an explanation of the hemispheric circulation anomalies that have led to this unusual North American cold in the midst of a warm hemisphere. At the same time, this set of circumstances makes the point that local weather and global scale climate change are not always in step with one another at a specific location.

Rather than appropriate unusual local weather conditions in a lazy and willfully unscientific argument about an important environmental issue, we encourage our public servants to examine the observations and the science surrounding them, ask difficult questions, and endeavor to seek the truth through careful analysis of the evidence.

Weather forecasters use different phrases to describe snowfall intensity:

• A light shower of snow that does not last long and covers a small region is called a snow flurry.

• When large amounts of snow fall, then the event is colloquially referred to as a snowstorm, though the term “large” is not officially defined.

• A snowstorm that lasts for three or more hours with sustained wind speeds of at least 35 mph and visibilities less than a quarter-mile is called a blizzard.

• A snowstorm that includes any occurrence of thunder is called a thundersnow event.

• A storm with freezing rain is referred to as an ice storm.

• Diamond dust, also called “clear-sky precipitation” is a cloud composed of tiny ice crystals that forms near the ground.

A weather watch informs us that current atmospheric conditions are favorable for hazardous weather such as a blizzard, heavy snow or heavy freezing rain. When the hazardous weather is occurring or will soon occur in an area, then a warning is issued. Weather warnings are issued when the event is a threat to life and property.

An advisory is a less urgent statement than a warning and is issued to bring the public’s attention to a situation that may cause some inconvenience or difficulty for travelers or people who have to be outdoors.